Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. In the field of imaging, the charge coupled device (CCD) sensor has made possible the manufacture of relatively low-cost and small hand-held video cameras. Nevertheless, the solid-state CCD integrated circuits needed for imaging are relatively difficult to manufacture, and therefore are expensive. In addition, because of the differing processes involved in the manufacture of CCD integrated circuits relative to MOS integrated circuits, the signal processing portion of the imaging sensor has typically been located on a separate integrated chip. Thus, a CCD imaging device includes at least two integrated circuits: one for the CCD sensor and one for the signal processing logic.
Some of the further drawbacks of CCD technology are discussed in "Active Pixel Sensors--Are CCD's Dinosaurs?" by E. R. Fossum, Proceedings of the SPIE--The International Society for Optical Engineering, Vol. 1900, 1993, pp. 2-14. As stated therein, "[a]lthough CCDs have become a technology of choice for present-day implementation of imaging and spectroscopic instruments due to their high-sensitivity, high quantum efficiency, and large format, it is well-known that they are a particularly difficult technology to master. The need for near-perfect charge transfer efficiency makes CCDs (1) radiation `soft,` (2) difficult to reproducibly manufacture in large array sizes, (3) incompatible with the on-chip electronics integration requirements of miniature instruments, (4) difficult to extend the spectral responsivity range through the use of alternative materials, and (5) limited in their readout rate."
An alternative low-cost technology to CCD integrated circuits is the metal oxide semiconductor (MOS), integrated circuit. Not only are imaging devices using MOS technology less expensive to manufacture relative to CCD imaging devices, for certain applications MOS devices are superior in performance. For example, the pixel elements in a MOS device can be made smaller and therefore provide a higher resolution than CCD image sensors. In addition, the signal processing logic necessary can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand-alone imaging device.
Examples of MOS imaging devices are detailed in "A 1/4 Inch Format 250K Pixel Amplified MOS Image Sensor Using CMOS Process" by Kawashima et al., IEDM 93-575 (1993), and "A Low Noise Line-Amplified MOS Imaging Devices" by Ozaki et al., IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991. In addition, U.S. Pat. No. 5,345,266 to Denyer, titled "Matrix Array Image Sensor Chip," describes a MOS image sensor. The devices disclosed in these publications provide a general design approach to MOS imaging devices. In addition, MOS approaches to color imaging devices are described in "Color Filters and Processing Alternatives for One-Chip Cameras," by Parulski, IEEE Transactions on Electron Devices, Vol. ED-32, No. 8, August 1985, and "Single-Chip Color Cameras With Reduced Aliasing" by Imaide et al., Journal of Imaging Technology, Vol. 12, No. 5, October 1986, pp. 258-260.
In the MOS solid-state color image sensors, a complementary color filter pattern is often used. Arrays of pixels may be made to detect color by being covered with a regular pattern of color filter patches, known as a color filter pattern. The filter patches can be fabricated directly on the sensor or on a transparent substrate which is later cemented to the chip. Color filter patterns may include colors such as red (R), green (G), blue (B), yellow (Ye), cyan (Cy) and magenta (Mg). The pixels beneath the color filter pattern emit signals when they are exposed to the type of light indicated by the color filter patch. Thus, a red signal could be obtained from a pixel beneath a red filter patch, a blue signal could come from a pixel beneath a blue filter patch, and so on.
However, some image sensors do not obtain the standard red, green and blue signals from red, green and blue filtered pixels. Instead, they use combinations of other colors to obtain the standard ones. For example, red (R) can be formed according to the equation R=(W+Ye)-(G+Cy), where the color filter pixel signals are W=white, Ye=yellow, G=green, and Cy=cyan. In cases such as this, the four pixel signals being processed are obtained by 2.times.2 block of one of each type of pixel sensors, rather than a 1.times.4 row of pixel sensors which would tend to distort the color image. The 2.times.2 block presents a problem for standard pixel scanning methods because standard methods scan each row, one at a time. In contrast, the 2.times.2 block of pixels comes from sections of two separate rows. Thus, the system cannot process the data as it scans each row. It must wait until the next row is also scanned to obtain the remaining information that it needs, and it must somehow save the data from the previous row until it does so.
Just as the color signals in such cases can be a combination of the signals from a 2.times.2 pixel block, the chrominance signal, which correlates to the color of the image, is also sometimes obtained from a combination of signals from pixels in two separate rows. In fact, this is the case for the chrominance signal in many systems, even those in which R, G and B filters are used to obtain the color signals directly. Therefore, it is required in such systems to somehow have the data from two separate rows available at the same time so that the required combinations can be processed. In most prior art devices, an external one-line delay line (e.g., a CCD delay line) is used for this purpose. The delay line scans in one row and holds the data until the next row can be scanned to provide the needed information.
In the movement from CCD- to MOS-based implementations, methods of implementing the circuitry have been sought that can easily be fabricated on a single MOS chip. The approach of using an external delay line device in the MOS color image sensors, which was carried over from the old CCD technology, has inherently required the use of components that are external to the MOS circuit and that are sometimes required to be on a separate chip, in addition to increasing the relative complexity of the implementation. It is a general principle that power consumption and cost would decrease if the number of chips and complex components necessary to accomplish the task were decreased. The present invention is directed towards a solution that provides the needed data from two rows of pixels at the same time, without using an external delay line device, and which can be integrated on a single-chip with the MOS sensor array.
In addition, the color filter patterns used in the prior art MOS color image sensors, from which the needed data is obtained, are also often not optimized. Any color can be thought of as a mixture of the three primary colors: red, blue, and green. However, as is known in the art, humans do not process red, green, and blue equally. Rather, humans rely upon the three primary colors in approximate accordance with: 0.6Green+0.3Red+0.1Blue. Thus, to the human eye, green is the most important color, red is the next most important, and blue is the least most important. Based upon this knowledge, the pixel array should have a color filter pattern (also referred to as a color coding scheme) that is more heavily weighted along the lines of green than red or blue. Prior art color coding schemes have attempted to achieve this in various ways.
The prior art has shown that checkerboard patterns are superior to linear ones. One of the most common schemes uses a matrix in which every other pixel is green (G), and the remaining pixels form a checkerboard pattern of alternating red (R) and blue (B). Another pattern uses a checkerboard pattern of equal amounts of cyan (Cy), yellow (Ye), white (W), and green (G). A noted advantage of using the colors cyan, yellow, and green is that since green is formed by overlapping yellow and cyan, only two filter fabrication mask steps are needed, as opposed to the three needed for a RGB filter. The present invention discloses a different color filter pattern, using green, yellow, and cyan, which is optimized for sensitivity.